1. Field of the Invention
The present invention generally pertains to capacitive measuring sensors, and in particular to capacitive measuring sensors for measuring force, torque, strain or torsion associated with a component upon which the sensor is fitted.
2. Discussion of the Related Art
Various types of capacitive torque or force sensors are known in the prior art. Torques and forces are currently generally measured in the industrial sector with the aid of measuring cells which are fitted with strain gauges, or the strain gauges are applied directly to the measuring point on a component. Because of the material combinations and the small size, high accuracy is required when sticking on strain gauges. This is very cost intensive, in particular in the case of series production or non-stationary applications such as, for example, in automobiles. In the case of strain gauges, the cemented joints exhibit great weaknesses with regard to long term performance, since their serviceability is strongly impaired by the influence of moisture, and it is not possible to ensure resistance to continuous loading.
Specific capacitive torque and force sensors are equipped with planar comb-shaped electrode structures which convert a force or a torque with corresponding strain or torsion into a change in capacitance. The displacements occurring in the process are only a few micrometers. In this case, the electrode spacing changes in accordance with a displacement at the measuring point. This application of a sensor is substantially simpler than strain measuring cells, and its long term stability is likewise substantially better. Likewise, this type of sensor can also be installed without difficulty on site, that is to say essentially in mobile units or systems.
It is generally a disadvantage of previous sensors that their electrode spacing is used as the measured variable, since the setting of small electrode spacing is problematical. Moreover, plane-parallel capacitor electrodes must be moved toward or away from one another while maintaining the mutual alignment. In the case of capacitive sensors, whose electrode structure is in the form of a comb, two comb structures fitted in a mutually insulated fashion must be mounted and guided parallel to one another to an extremely high degree. Furthermore, high accuracies are required in the division when producing the comb structures. These aspects have high production costs, even in batch-quantity runs. Furthermore, the minimum electrode spacing, and thus the sensitivity of the sensor principle are limited by tolerances owing to mounting.
Japanese Patent Publication JP-A-57/042829 discloses a pressure transducer which measures the input pressure on a diaphragm with high accuracy by capacitively detecting a corresponding relative movement between two electrodes.
European Patent Application EP-A-0 759 628 describes a variable capacitor having a movable region. Upon appropriate variation in the movable region, an element is displaced in such a way that a variation in the capacitance of the capacitor occurs. The capacitance set is fixed in each case, and the capacitor does not serve to take up measured values.
The present invention departs from prior art capacitive sensor principles, in which variations in the spacing of plane-parallel electrodes is evaluated. Instead, the present invention uses variations in the angles of electrodes or electrode surfaces arranged at an acute angle or in a wedge-shaped fashion relative to one another as the sensing.
Measuring signals, in particular variations in capacitance, can be determined in order to determine distances/displacements, torsions, and strains or forces and torques. Thus, the sole precision technology required is in the production of very plane active electrode surfaces which are opposite one another and cooperate capacitively.
The overall arrangement of the present invention can be designated as a wedge or a wedge-shaped structure. Due to the fact that capacitance faithfully obeys the relationship c≈1/d, where c=capacitance and d=electrode spacing, sensors using this principle need not be calibrated.
Therefore, a sensor arrangement has extremely low stray capacitances and low capacitances of second order. Temperature dependence of capacitive sensors, which is already very weak, is improved by use of an angle spacing which is absolutely uniform, and is measured is a capacitance, or its reciprocal, without regard to changes in electrode spacing caused by unwanted variation.
The sensor of the present invention is elongated and partially plate-shaped. It is connected at its end regions by means of a two-point connection, that is to say a force-close connection at each end to a body, for example a shaft, to be measured. Orientation of the elongated sensor relative to the body to be measured is such that displacements owing to strains or torsions offset against one another at the fastening points of the two-point connection between the sensor and the body to be measured. A first electrode is connected to an object to be measured by means of a fastening element. A support for a second electrodes is likewise rigidly connected to this fastening element, and the rigid support is therefore not directly connected to the body to be measured. The first electrode is plane in design in its middle region between the fastening points, and is not situated parallel opposite the second electrode, which is located on the support.
The first electrode is designed to be metallically conducting only on the surface. The second electrode is expediently mounted on an insulating support. All the electrodes are constructed to be as plane as possible on their active surface.
The advantages of such a capacitive measuring sensor include a high output capacitance in conjunction with a low overall volume. Furthermore, there is a high linearity (1/C), and temperature drift of the zero point is very slight, thermally induced strains are minimal and the measuring sensor requires no complicated adjustment, such as precision adjustment of electrodes, during mounting. In addition, necessary manufacturing tolerance are slight with regard to the response characteristic, therefore, there is no need for calibration.
The parts of the capacitive sensor which constituted the electrodes are produced using thin-layer technology in order to achieve a high-precision planarity, and the first electrode is of single design, while the two second electrodes cooperate capacitively opposite one another on the support.
An integrated circuit (IC), which outputs a signal representing the capacitance or the reciprocal thereof, because of its small size, can be fitted on the support for the purpose of logging measured values. The IC can also be used for data transmission of the capacitive measurements. The sensor can preferably be surrounded by a housing in order to protect against aggressive and harsh environmental influences.
If the surface of the electrodes is of plane design, and an imaginary point of intersection of the angularly set electrodes or electrode surfaces equates to the center of rotation for variations in angle, a simple mathematical relationship applies for changes in spacing of electrodes with regard to a change in capacitance.
Displacements or distances, torsions, strains, forces and torques can be measured reliably with the aid of such a measuring sensor.
It is an object of the invention to provide a capacitive measuring sensor of simple design having an improved sensibility.
It is another object of the invention to provide a capacitive measuring sensor that uses variations in the angle of electrodes or electrode surfaces arranged at an acute angle or in a wedge-shaped fashion, relative to one another, as sensing elements.
It is an additional object of the invention to provide a capacitive measuring sensor having electrodes that use thin-layer technology in order to achieve a high-precision planarity.